The up-conversion from baseband to radio frequency (RF) in radio transmitters can be performed as a single step “direct conversion” using an analog IQ-modulator architecture or as a multiple step “digital intermediate frequency” conversion using both digital and analog mixers. Generally the IQ-modulator architecture is preferable, since it gives better performance and is less complex. However, this also requires a simple and effective way of correcting for the so-called IQ-errors generated by the IQ-modulator.
Conventionally the IQ-errors are compensated in an IQ-error compensator, the parameters of which are determined by an adapter using the baseband signal and a signal from a so-called observation receiver. This observation receiver converts the radio frequency (RF) signal from the power amplifier to baseband in two steps. In the first step an analog mixer converts it to intermediate frequency, where it is digitized. The digitized intermediate frequency signal is then digitally converted to an IQ-signal at baseband by a digital mixer. A drawback of the known method, however, is that several analog local oscillators have to be used, one for the IQ-modulator and one for the observation receiver. In addition to the increased costs, this approach leads to a complicated cancellation of the phase noise of the transmitter and observation receiver local oscillators. There is also a risk for leakage from the observation receiver local oscillator.